The printed circuit board (PCB) is an element in many electronic products that contain electronic components. The PCB holds components and connects these components electrically through metal traces. In some applications, especially high speed serial data communications, high quality traces are required. Thus the characteristics of these metal traces must be measured and tested to ensure high quality.
Until relatively recently, the determination PCB and PCB trace quality meant the determination of the ability to handle current or voltage and or electrical resistance. As of late, however, many high-speed PCB traces are tested for characteristic impedance and very lately, methods have been proposed and utilized for measuring loss.
Characteristic impedance is a characteristic of a trace that is dependent on dimensions and material characteristics of the trace and is an electromagnetic aspect that is usually interesting only at relatively high frequencies. It is defined as the square-root of the ratio of inductance to capacitance for a given unit length of trace. When elements in the path of a high-speed signal have different characteristic impedances, reflections occur which may degrade communications. When a trace is excessively lossy signals may be attenuated at high frequencies and again, the communications capability is degraded.
As trace characteristics are dependent both on dimensions and material characteristics of the trace and board, the traditional method of maintaining quality has been for PCB manufacturers to maintain dimensional, mechanical, and chemical property specifications, and for the PCB customers to choose materials and specify dimensions that ensure the desired characteristics. These traditional methods have been less successful as of late, especially when the traces are to be used with high speed data transfers. Many PCB customers and even silicon chip producers to PCB customers are demanding electrical trace characteristics measurements during board construction.
There are several problems with this situation. First, for many PCB manufacturers, high-frequency electrical characteristics are a domain of engineering that is foreign to them. Next, the measurement of high-frequency characteristics are difficult and often time consuming. Finally, the equipment needed to perform these measurements is very expensive.
The industry needs methods that are fast, easy, and economical.
Driven by this need, companies have produced instruments, probes, algorithms and software for these measurements and standards bodies have created measurement standards and methods such as IPC-TM-650 available from the Institute for Interconnecting and Packaging Electronic Circuits (IPC).
Recently, a method has been created and proposed for making measurements called single-ended to differential insertion loss (SE2DIL) (also called SET2DIL). The details of this measurement have been provided in J. Loyer & R. Kuntze, “SET2DIL: Method to Derive Differential Insertion Loss from Single-ended TDR/TDT Measurements”, DesignCon, 2010. This method provides for measurement of differential insertion loss by taking a single single-ended measurement of a differential trace with a specified structure. The method prefers time-domain reflectometry (TDR) and time domain transmission (TDT) methods, but it is well known that equivalent vector network analyzer (VNA) measurements can be used by simply converting the measured s-parameters from the VNA to the time-domain. TDR/T instruments tend to be less expensive and more familiar to PCB manufacturers. VNA instruments tend to be more expensive and less familiar to PCB manufacturers. Because of the ability to measure loss using a single single-ended two-port measurement, the method is relatively fast and requires only a two-port VNA or TDR instrument, which is really the key aspect as the measurements facilitated by SE2DIL could be made easily with a four-port instrument and a four-port measurement test structure, albeit with more test instrument cost, complexity and time.
Section 2.5.5.12 of IPC-TM-650 provides various test methods to determine the amount of signal loss on printed circuit boards. Currently, there are four methods recommended for determining such signal loss, with SE2DIL being proposed as a fifth method.
One problem with the SE2DIL method is that it is somewhat of an adhoc, algorithmic approach. It has been found that there are many parameters in the algorithm that must be tweaked to get good repeatable results and that there are dependencies on characteristics of measurement instruments that are not entirely understood. Despite this, empirical tests have been performed and indications are that it performs satisfactorily. A method with mathematic and measurement rigor is, however, preferable and a new method meeting this criteria would be more useful. While other methods exist for fully determining electrical characteristics of PCB traces, SE2DIL concerns itself with loss only.
What is needed is a method with mathematic and measurement rigor that can determine more PCB trace characteristics that also has the fast, inexpensive and easy characteristics of SE2DIL.